1. Field of the Invention
The present invention relates to a device for analyzing the function of a heart, having a measurement unit for generating a measurement signal related to an electrical or mechanical heart variable, and an evaluation unit for evaluating the measurement signal.
2. Description Of the Prior Art
In the monitoring, diagnosis and treatment of a heart's function, accurate determination of the heart's current condition, with minimal risk of erroneous interpretations, is important. Automatic monitoring of the heart is a valuable asset in the treatment of heart disease so that a therapeutic measure can be instituted without delay when necessary.
The electrocardiogram (ECG) is one heart variable which is an indicator of a heart's function. Sensing the ECG in order to obtain a measurement signal which can be evaluated in establishing the condition of the heart is known in the art.
One way to graphically elucidate the electrocardiogram by plotting the voltage in a recorded electrocardiogram against the time derivative of the voltage is described in an article entitled "Phase Plane Plot of Electrograms as a Marker of Ventricular Electrical Instability During Acute Ischemia: Initial Experimental Results and Potential Clinical Applications", published in the journal PACE, Vol. 15, part II, November 1992, pp. 2188-2193. This procedure produces a curve corresponding to the ECG signal. The article shows that there is a relationship between changes in parts of the curve during acute ischemia and the development of ventricular fibrillation. The authors of the article state that a presentation of an electrocardiogram in graphical form can be an excellent complement to traditional, real-time presentation.
U.S. Pat. No. 4,417,306 describes an apparatus which monitors and stores heart signals. The apparatus senses the ECG, and the ECG signal must have a predesignated slope, amplitude, duration and course to be accepted as a heart beat. The QRS complex is the main segment sensed, i.e., the electrical signals which occur in the heart when there is a ventricular beat (ventricular systole).
U.S. Pat. No. 4,453,551 describes an apparatus designed to detect ventricular fibrillation (VF). The apparatus senses the ECG signal from the heart, digitizes it and amplifies it to a predetermined amplitude. The amplified signal can then be analyzed in different ways to ascertain whether or not VF is present. For example, the statistical distribution of gradients or the frequency of the maximum negative gradient can be analyzed.
European Application 0 220 916 describes an apparatus designed to detect the presence of ventricular tachycardia (VT) and VF and to supply treatment to terminate these conditions. The apparatus senses the heart's ECG at a plurality of points on the heart and determines the sequence in which the signals are detected at the different measurement points. In VT and VF, the sequence deviates from the normal pattern in different ways.